No matter what, people always feel like their paying way too much and are usually dicks about it.
I've never done it myself but hear horror stories and how annoying wedding people can be.
4 or 5 of them trying to tell you how to shoot the photo's and giving you shit like they know how to shoot better than the photographer.
You have to hold all the pics until it's payed in full also, any chance they can get they won't pay you.